Recently issued U.S. Pat. No. 4,759,291 sets forth a device for initiating detonation of a detonating cord. The present disclosure is directed to improvements for use in a deep well detonation system. It is particularly intended for use in deep oil or gas wells where perforations are normally formed into the adjacent producing formations. The perforations involve several explosive components, the explosive train typically including a firing pulse generator (FPG), a detonator, a connected detonating cord and one or more shaped charges. The shaped charges are spaced along the tool which supports all of this equipment. Dependent on the location of the well and other factors, the explosive equipment may be exposed to temperatures as high as about 500.degree. F. at ambient pressures in excess of 20,000 psi. Clearly, this high temperature and pressure places an extraordinary stress on the explosive train, and particularly on the detonating cord which is encased circumferentially in a jacket and where the explosive in the cord is subjected to both high temperature and pressure. In general terms, elevated temperature and pressure is detrimental to the explosive components. That is, the components degrade as a function of temperature and pressure. It is necessary to utilize only thermally stable explosive compounds in the explosive train. Representative compounds are PYX or ONT, two explosives which have excellent thermal stability. These explosive materials are relative easy to manufacture and are reasonable cost products. They have excellent thermal stability and are able to tolerate temperatures as high as 500.degree. F. for 100 hours. Generally, there are limits to all explosive materials because they are subject to accelerated degradation when exposed to high pressures and temperatures.
It has been ascertained that a detonating cord made from these explosives is highly desirable, but one of the features which makes it highly desirable also makes it difficult to detonate. Important features involved in the design of an explosive train featuring such a detonating cord include the inherent thermal stability of these reactive materials; this yields an explosive train which is quite insensitive both to heat and shock. This insensitivity increases the requirements necessary to detonate such explosives. Moreover, the extraordinary pressures encountered in deep boreholes increase the packing or density of the explosive materials inside the detonating cord and thereby decreases sensitivity. Also, the particle size of the explosive material which is most suitable for detonating cord fabrication is not conducive to easy initiation. As a clear result of these conflicting requirements, the detonating cords made of such thermally stable explosive materials when installed in a downhole environment are extremely difficult to initiate.
Conventional detonators do not provide adequate shock strength for initiation of such detonation cords. The detonator which is set forth in the referenced patent may well encounter difficulty in initiating an explosive cord in such circumstances. The apparatus in accordance with the teachings of the present disclosure overcomes these and other limitations. It is desirable to provide a crimped structure that affixes to the end of a detonating cord and which has a transverse barrier which prevents the end of the detonating cord from being extruded when subjected to extraordinarily high pressures. This barrier is incorporated for the express purpose of confining the detonating cord. However, the imposition of a barrier across the housing impedes detonation shock wave transfer to the detonating cord. The present disclosure sets out a device which overcomes transfer problems associated with a transverse barrier. Another important feature of the apparatus affixed to the detonating cord is the incorporation of a small cylinder or pellet of pressed secondary explosive material on one side of the barrier. When firing does occur, the secondary explosive assists in transferring the shock wave to the detonation cord through a transverse barrier. There are further means including a retainer shoulder for registration purposes and a surrounding peripheral crimp which is included to assure that the equipment holds together under such severe circumstances.
An important feature of the present disclosure is overcoming shock impedance mismatches involved in the relative changes of impedance to the shock wave transmitted through the barrier. The transverse barrier is preferably made of aluminum or similar metal for structural reasons; however, the shock wave which traverses a metal transverse member will inevitably lose shock pressure when thereafter directed into the adjacent detonating cord, a lower impedance material. The present disclosure contemplates a specially shaped and designed transverse barrier or bulkhead. That is, it is constructed in the form of a concave region to thereby define a concave lens. This lens system focuses the shock waves transmitted thereby, and the focused waves thus impinge after traversing the barrier in a specified region. Ideally, this region includes a relatively short cylindrical pellet of special explosives as will be set forth, and that in turn is abutted against the end of the detonating cord. This focusing arrangement works quite well to provide a boost in shock wave intensity. A further boost can be obtained by arranging three such pellets adjacent to the transverse bulkhead where each directs the shock wave along the axis of the crimped structure across the barrier, and then overlapping in a common focal region. Thus, three such pellets can be used wherein each forms its own shock wave which is transmitted across the transverse bulkhead; the shock waves formed thereby overlap in an adjacent region. As all the shock wave energy is brought to that region, the shock impulse necessary to accomplish detonation is thereby achieved. One feature is the secondary explosive initiating compounds are located at or in the region of the converging shock waves so that they, notwithstanding their extraordinary stability, are initiated properly and then convey the initiation shock wave to an adjacent detonating cord termination. This properly directs the explosive shock wave into the cord where it is desired.
In one particular embodiment of the present disclosure, there is a gap with an air space between, and a flyer or plate is propelled across the space to deliver a shock wave to the facing area. This flyer impact causes a shock wave of substantial amplitude. The shock wave is further magnified by incorporating a recessed, high impedance reflector plate within the initiating compound and adjacent to the end of the detonating cord. This forms a reflective surface which reflects a higher pressure wave, thereby increasing the shock level so that sufficient shock is present for detonation. The preferred and alternate embodiments will be described in detail hereinafter on review of the written specification in conjuction with the attached drawings.